It has been known for a relatively long time to use an insulating material that is printed on both sides with epoxy resin for the purpose of insulating windings of electrical machines (e.g. transformers, transducers). Typically, said insulating material has a diamond-shaped epoxy resin coating. In technical terminology said insulating material is referred to as “Diamond Pattern Epoxy Coated Paper” (DPP).
An insulating material containing an insulating layer made of an organic polymer is known from European patent application EP 0 287 814 B1. Said European patent application discloses an insulating material having an improved corona resistance, without an increase in the overall thickness of the insulating material being necessary for this.
If the insulation frays or tears, corona discharges can occur between the individual turns of the winding, thereby reducing performance and negatively affecting the service life of the wound coil assembly.
The object of the present invention is to provide an insulating material which offers a high degree of stability in the insulating state.
This object is inventively achieved in the case of an insulating material of the kind cited in the introduction in that the adhesive coatings are applied and aligned on the front and back sides of the carrier material in such a way that in the carrier material a direct frictional connection is developed in the insulation state between the adhesive coatings on the front and back of the carrier material. If a direct frictional connection exists between the points of adhesion of the adhesive coatings on the front and back sides of the carrier material following the baking process (or following the bonding), no shearing forces or only minimal shearing forces are produced between the points of adhesion of the front and back sides. Furthermore, the carrier material (e.g. kraft paper, pressboard, aramid paper, polyester film, textile fiber, or a textile-based material) is also not exposed to any substantial force stress. This has a positive effect on the useful life of the insulating material. In particular, fraying and shearing of the insulating material is prevented by the direct frictional connection, which in turn has a positive effect on the reliability and operational safety of the electrical machine. Because shearing forces are avoided as a result of the direct frictional connection between the points of adhesion on the front and back sides, a high degree of stability is present in the baking (or bonding) of the windings. Moreover, less adhesive material (epoxy resin or another suitable adhesive) is required for the adhesive coatings and in addition the high degree of stability of the baking (or bonding) permits the use of a thin carrier material, thereby reducing the costs of the insulating material. Furthermore, a high degree of stability of the baking prevents a fraying or shearing of the insulating material, thereby avoiding corona discharges between the individual turns of the wound coil assembly.
According to a first advantageous embodiment of the invention, epoxy resin is used as the adhesive material for the adhesive coatings. Epoxy resin as an adhesive material is easy to handle and to apply to the carrier material. Furthermore, epoxy resin possesses advantageous thermochemical properties. Thus, at the time of shipment on the carrier medium the epoxy resin is disposed in what is termed a reactive B-state. Upon being heated to higher temperatures the epoxy resin becomes adhesive and bonds to the turns of the electrical conductors. After a curing time has elapsed the epoxy resin assumes what is termed a C-state and an infusible stable bond exists with the turns of the electrical conductors. The thickness of the epoxy resin coating lies in the region of approx. 0.020 mm per side, depending on the carrier medium.
According to a further advantageous embodiment of the invention, the adhesive coatings on the front and back sides of the carrier material are in each case arranged congruently or virtually congruently with respect to each other. It is thereby ensured that the mechanical loading of the carrier material induced by the adhesive forces is minimized, since no shearing forces are produced in the carrier material owing to the adhesive coatings being disposed opposite each other in each case. Tears and fraying in the carrier material can occur due to shearing forces. The useful life of the insulating material is therefore increased as a result of the avoidance of shearing forces.
According to a further advantageous embodiment of the invention, the adhesive coatings on the front and back sides of the carrier material represent an essentially opposite symmetrical pattern in each case. This arrangement of the adhesive coatings ensures a direct frictional connection between the points of adhesion of the adhesive coatings on the front and back sides of the carrier material. Consequently, no shearing forces act between the points of adhesion on the front and back sides of the carrier material. The opposite symmetrical arrangement of the points of adhesion on the front and back sides of the carrier material in each case ensures that the mechanical loading induced on the carrier material by the adhesive forces is minimized. Among other things, this increases the useful life of the carrier material and therefore also of the insulating material.
According to a further advantageous embodiment of the invention, the adhesive coatings are diamond-shaped. Diamond-shaped adhesive coatings constitute a partial coating on the front and back sides. The partial coating leads to a partial bonding of the turns of the electrical conductors to the insulating material. Consequently there are unglued free spaces present between the insulating material and the turns of the electrical conductors which make a positive contribution to the drying and impregnation process. Moreover, diamond-shaped epoxy resin coatings on transformer pressboard or on kraft paper are also suitable for withstanding the high power levels and voltages in active parts of transformers (in particular power transformers) or transducers, in particular with regard to dielectric or mechanical stresses. A suitable side length of the diamonds lies in the range of 9-10 mm, while a suitable distance between two diamonds (distance between two adjacent side lengths) lies in the range of 5.5-7 mm.
According to a further advantageous embodiment of the invention, the adhesive coatings are circular in shape. The advantage of circular or dot-shaped adhesive coatings lies in the fact that they can easily be applied to the carrier material by pressure or using spray-on techniques. In principle, however, any shape is suitable as long as it is ensured that the adhesive coatings on the front and back sides of the carrier material are positioned congruently opposite each other or are arranged essentially congruently on the front and back sides of the carrier material.
According to a further advantageous embodiment of the invention, transformer pressboard, kraft paper, aramid paper, polyester film, textile fiber or a textile-based material is used as the carrier material. Said materials possess a high degree of deformability and flexibility and are therefore easy to handle. Transformer pressboard and kraft paper additionally possess high electrical breakdown resistance and great tensile strength and consequently can be exposed to extreme voltage stresses. Aramid paper (e.g. NOMEX®) is particularly suitable for satisfying high thermal and dielectric requirements. Textile fibers and textile-based materials are inexpensive and suitable in particular for small voltage ranges.
According to a further advantageous embodiment of the invention, the electrical machine is a transformer, a converter, a transducer or a generator. Extreme dielectric and mechanical stresses often exist in active parts of electrical machines such as e.g. transformers, frequency converters, transducers or generators, adversely affecting the useful life and operational efficiency of the conductor insulation used. If a direct frictional connection is present in the carrier material between the adhesive coatings of the front and back sides, the shearing forces in the carrier material of the insulation are avoided or reduced, thereby resulting in a positive effect on the useful life, operational efficiency and maintenance of the insulation.
A further advantageous embodiment of the invention consists in the use of the inventive insulating material for insulating turns of wound coil assemblies in electrical machines. The service life of power transformers is described in particular by the useful life of the insulation system, since in the event of a mechanical instability of the insulating material there is an increased risk of a dielectric failure and an attendant potential total failure of the transformer. Aging effects and the mechanical and electrical load on the carrier material play a critical role in terms of the useful life of the insulation system. If the carrier medium is not exposed in the insulation state to any shearing forces due to the points of adhesion on the front and back sides, this has a positive effect on the useful life and operational efficiency of the entire insulation system.